1. Field of the Invention
The present invention relates to an electric tightening device for tightening a bolt to a member as an object of tightening such as a steel frame.
2. Background of the Invention
FIG. 7 is a side view showing bolts 90 and nuts 9 for building or bridge temporarily attached in a plurality of positions in a member 91 as an object of tightening such as a steel frame and a conventional electric tightening device 8 for tightening the nuts 9 to the bolts 90 one by one (refer to Japanese Patent Laid-Open No. 2000-117649). The electric tightening device 8 is constructed by housing a motor M and a planetary gear mechanism 82 including a reduction gear in a tightening device body 80 and coupling an outer socket 7 to the tip of the tightening device body 80. A handle 81 of the tightening device body 80 is provided with a switch SW for starting/stopping operation.
The outer socket 7 integrally has a projected reaction force receiving arm 70 on its outer periphery and has therein an inner socket 6 which engages with the nut 9 and rotates the nut 9. In the planetary gear mechanism 82, the rotary shaft of the motor M is coupled to a center gear (sun gear), the inner socket 6 is coupled to a plurality of planetary gears engaged with the center gear, and the outer socket 7 is coupled to a revolution gear engaged with the planetary gears. When the inner socket 6 is engaged with the nut 9 and the motor M is rotated by turning on the power source, the reaction force receiving arm 70 is rotated by the planetary gear mechanism 82. From the time point when the reaction force receiving arm 70 comes into contact with the neighboring nut 9a, the inner socket 6 rotates and the nut 9 accordingly rotates, thereby tightening the nut 9.
Electric power is fed to the motor M from a battery (not shown) as a power source. When the residual quantity of the battery is insufficient, it is indicated by a pilot lamp or the like. The power source is not limited to a battery but may be a DC or AC power source from the outside. In the following, the case where a battery is used as a power source will be described. In the case of the AC power source, “shortage of the residual quantity of the battery” corresponds to “insufficient capacity of the AC power source or an electric circuit”.
There are various means for determining whether the residual quantity of the battery is insufficient or not. When the determination is made by detecting only a terminal voltage of a battery before the motor M is started, the following problem occurs.
Specifically, even when the terminal voltage before the motor M is started is sufficient, for example, in a deteriorated battery, a voltage drop of the battery terminal voltage upon start of the motor M is larger than that in a less-deteriorated battery.
Therefore, in the deteriorated battery, even if the battery terminal voltage before the motor M is started is a voltage which can start the motor M, when the nut 9 is tightened and a load increases, the battery terminal voltage largely drops, malfunction occurs during operation of the motor M, and there is the possibility that the nut 9 cannot be tightened properly.
There is another electric tightening device in which battery temperature measuring means, means for storing the residual quantity of a battery and, further, means for entering a discharge efficiency value preliminarily obtained by experiments into a data table and computing an exact battery residual quantity on the basis of a value read from the data table are assembled. However, the electric tightening device has a problem that the cost is too high.
An object of the present invention is to accurately notify the operator of whether a nut can be tightened or not with a simple configuration that does not include the battery temperature measuring means, a data table, and the like.